1. Field of the Invention
The present invention relates to a carbon fiber-reinforced carbon composite material which is composed of a matrix of carbonaceous material and a reinforcing material of carbon fiber in the form of nonwoven fabric.
2. Description of the Prior Art
Carbon fiber-reinforced carbon composite material is referred to as C/C composite material. It is used as a heat-resistant material in the fields of spacecraft and aircraft. The C/C composite material is produced in the following manner. At first, a preform of nonwoven fabric or woven fabric is impregnated with pitch or a thermosetting resin such as phenolic resin, furan resin, and epoxy resin, thereby to form a prepreg. A plurality of the prepregs are laminated on top of the other to form a multi-layer product. The layers are made into a unified body by press molding or the like. The unified body is calcined so that the matrix resin is carbonized and graphitized. If necessary, the impregnation and calcination are repeated to increase the density of the composite material. The density increase is also accomplished by chemical vapor deposition (referred to as CVD hereinafter).
Increasing the density to a desired value by repeating impregnation or by performing CVD takes a long time because closed voids are formed in the matrix (depending on the type of the matrix carbon) or pores are nearly closed to prevent the impregnation.
Hard carbon is usually made from a thermosetting resin. A disadvantage of hard carbon is that it forms voids while it undergoes impregnation repeatedly and voids prevent the increase of density. By contrast, soft carbon is made from a thermoplastic resin, pitch, or coke. A disadvantage of these materials is that their conversion into carbon is usually low, e.g., 60% at the highest, and repetition is required to achieve the desired conversion. An additional disadvantage is that they do not permeate small pores easily.
One disadvantage involved in C/C composite materials is that an impregnant or coating fluid such as thermosetting resin does not infiltrate into individual yarns constituting the nonwoven fabric or woven fabric. Each yarn is a collection of 1000 to 4000 fine long filaments. This leads to delamination and cracking at the interface between the matrix and the reinforcement fiber that take place after carbonization and calcination. Another disadvantage of C/C composite materials is that there is locally unbalanced strength which causes delamination and deformation during the use of C/C composite material.
In order to eliminate these disadvantages, there was developed a C/C composite material produced from a prepreg composed of a matrix of thermosetting resin and a reinforcing material of short carbon fiber. Unfortunately, this C/C composite material is poor in strength and impact resistance because the fiber-to-matrix ratio is small and individual fibers are arranged apart. An additional disadvantage of the C/C composite material based on a short fiber reinforcing material is that the strength in the direction parallel to the surface is low as compared with that in the direction perpendicular to the surface.
A C/C composite material used as a friction material is required to have not only frictional properties but also high strength, oxidation resistance, and thermal conductivity, because it receives texture, compression, and shocks during its use. However, frictional properties are not compatible with strength or oxidation resistance. For a C/C composite material to have improved frictional properties, it should have a crystalline structure, which is formed by heat treatment (or graphitization) at 2500.degree. C. or above.
The relationship between the heat treatment temperature and the characteristic properties (strength, frictional properties, and oxidation resistance) is graphically shown in FIG. 5. It is noted from FIG. 5 that raising the heat treatment temperature to improve the frictional properties lowers the mechanical strength. Thus it is difficult to improve both the frictional properties and the strength at the same time.
By the same token, raising the heat treatment temperature improves oxidation resistance but lowers strength on account of the carbonization accompanied by increased pores. Thus the improvement of oxidation resistance by heat treatment alone is limited. According to the prior art, the desired oxidation resistance was achieved by the application of an oxidation resistant coating instead of heat treatment at high temperatures.
The C/C composite material is composed of carbon fiber as a reinforcing material and carbon as a matrix. The carbon fiber as a reinforcing material is broadly classified into PAN (polyacrylonitrile)-based one and pitch-based one. The former is in general use where high strength is required. A disadvantage of PAN-based carbon fiber is that it is not wetted by the matrix very easily. Therefore, surface modification at the sacrifice of strength is under study.
Since carbon fiber is used in the form of tow, PAN-based carbon fibers having a low modulus are made into a fiber bundle by the aid of a sizing agent. This sizing agent deteriorates the wettability of the carbon fibers.
The impregnation of the matrix is limited by many factors. First, the impregnant is required to contain no granular, solid, or gel-like substances. Second, the impregnant is required to have a low viscosity and to be highly compatible with carbon fibers or carbon fiber bundles.